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General Relativistic Evolution Equations for Density Perturbations in Closed, Flat and Open FLRW Universes

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General Relativistic Evolution Equations for Density Perturbations in Closed, Flat and Open FLRW Universes General Relativistic Evolution Equations for Density Perturbations in Closed, Flat and Open FLRW Universes P. G. Miedema∗ Netherlands Defence Academy Hogeschoollaan 2 NL-4818CR Breda The Netherlands March 9, 2021 Abstract It is shown that the decomposition theorems of York, Stewart and Walker for symmetric spatial second-rank tensors, such as the perturbed metric tensor and perturbed Ricci tensor, and the spatial fluid velocity vector imply that, for open, flat or closed Friedmann-Lemaître-Robertson-Walker uni- verses, there are exactly two, unique, independent gauge-invariant quantities which describe the true, physical perturbations to the energy density and particle number density. Using these two new quanti- ties, evolution equations for cosmological density perturbations and for entropy perturbations, adapted to non-barotropic equations of state for the pressure, are derived. Density perturbations evolve adia- batically if and only if the particle number density does not contribute to the pressure. Local density perturbations do not affect the global expansion of the universe. The new perturbation theory has an exact non-relativistic limit in a non-static universe. The gauge problem of cosmology has thus been solved. pacs: 04.25.Nx; 98.80.-k; 98.80.Jk keywords: Cosmology; Perturbation theory; mathematical and relativistic aspects of cosmol- ogy arXiv:1410.0211v4 [gr-qc] 7 Mar 2021 ∗[email protected] 1 Contents 1 Introduction 2 1.1 Gauge Problem of Cosmology . .3 1.2 Previous Attempts to solve the Gauge Problem of Cosmology . .3 1.3 Solution of the Gauge Problem of Cosmology . .4 2 Preliminaries 5 2.1 Choosing a System of Reference . .5 2.2 Equations of State for the Pressure . .6 3 Einstein Equations and Conservation laws for Closed, Flat and Open FLRW Universes6 3.1 Background Equations . .6 3.2 Linearized Equations . .7 4 Decomposition of the Spatial Metric Tensor, the Spatial Ricci Tensor and the Spatial Fluid Velocity8 4.1 Tensor Perturbations . .8 4.2 Vector Perturbations . .9 4.3 Scalar Perturbations . .9 5 Linearized Equations for Scalar Perturbations in Closed, Flat and Open FLRW Universes9 6 Unique Gauge-invariant Quantities 11 7 Non-relativistic Limit 12 8 Evolution Equations for the Density Contrast Functions 15 9 Thermodynamics 17 9.1 Pressure and Temperature Perturbations . 17 9.2 Diabatic Density Perturbations . 18 9.3 Adiabatic Density Perturbations . 18 10 Conclusion 19 A Derivation of the Evolution Equations using Computer Algebra 20 A.1 Derivation of the Evolution Equation for the Energy Density Perturbation . 20 A.2 Derivation of the Evolution Equation for the Entropy Perturbation . 22 A.3 Evolution Equations for the Contrast Functions . 23 1 Introduction The theory related to the linearized Einstein equations and conservation laws is important in cosmology because it describes the growth of all kinds of structures in the expanding universe, such as stars, galaxies and microwave background fluctuations. Lifshitz(1946) and Lifshitz and Khalatnikov(1963) were the first researchers to derive a cosmological perturbation theory. They encountered the problem that the solutions of the linearized Einstein equations and conservation laws have no physical significance. Due to 2 the linearity of the equations, the solutions can be changed by a linear coordinate transformation, i.e., a gauge transformation. Therefore, the solutions are gauge-dependent. This is the well-known gauge problem of cosmology which is up till now not solved. 1.1 Gauge Problem of Cosmology Consider a closed, flat or open Friedmann-Lemaître-Robertson-Walker (flrw) universe filled with a perfect fluid, with an equation of state for the pressure which depends, according to thermodynamics, on the particle number density n and the energy density ". The evolution of the energy density "(0) and the particle number density n(0) of the background universe are governed by the usual Einstein equations and conservation laws for a flrw universe. The general solution of the linearized Einstein equations and conservation laws contains, among other quantities, "(1) and n(1). Since the linearized equations are invariant under linear coordinate transformations xµ ! x0µ = xµ − ξµ, one can generate new, equivalent, 0 0 solutions "(1) and n(1), i.e., 0 0 0 0 "(1) = "(1) + ξ "_(0); n(1) = n(1) + ξ n_ (0); (1) 0 0 where ξ "_(0) and ξ n_ (0) are gauge modes. Therefore, "(1) and n(1) are gauge-dependent and have, as a consequence, no physical significance. In contrast to "(1) and n(1) the real, measurable density perturbations phys phys "(1) and n(1) are independent of the choice of a coordinate system, i.e., are gauge-invariant. The physics of density perturbations is hidden in the general solution of the linearized Einstein equa- phys phys tions and conservation laws. Therefore, the physical quantities "(1) and n(1) can be expressed as linear combinations, with time-dependent coefficients, of gauge-dependent solutions of these equations, such that phys phys the gauge modes are eliminated. Furthermore, "(1) and n(1) have the property that in the non-relativistic phys phys limit the linearized Einstein equations and conservation laws combined with expressions for "(1) and n(1) phys reduce, in a non-static flat flrw universe, to the time-independent Poisson equation with "(1) as source phys phys 2 term, and the well-known Einstein relation "(1) = n(1) mc . Consequently, the gauge problem is, in fact, phys phys the problem of finding expressions for "(1) and n(1) . 1.2 Previous Attempts to solve the Gauge Problem of Cosmology Bardeen(1980) was the first to demonstrate that using gauge-invariant quantities, one can recast the linearized Einstein equations and conservation laws into a new set of evolution equations that are free of non-physical gauge modes. Bardeen’s definition of a gauge-invariant density perturbation is such that it becomes equal to the gauge-dependent density perturbation in the limit of small scales. The underlying assumption is that on small scales the gauge modes vanish so that gauge-dependent quantities become phys phys gauge-independent. As will be shown in Section7, the physical quantities "(1) and n(1) do not become equal to the gauge-dependent quantities "(1) and n(1) in the non-relativistic limit. Furthermore, the gauge modes "^(1) and n^(1) never become zero, since the universe is not static in the non-relativistic limit. Finally, the relativistic space-time gauge transformations reduce in the non-relativistic limit to Newtonian gauge transformations with space and time decoupled. Consequently, the quantities defined by Bardeen are not equal to the real physical energy density perturbation. The article of Bardeen has inspired the pioneering works of Ellis and Bruni(1989); Ellis et al.(1989); Ellis and van Elst(1998), Mukhanov et al.(1992); Mukhanov(2005) and Bruni et al.(1992). These researchers proposed alternative perturbation theories using gauge-invariant quantities which differ from the ones used by Bardeen. The theory of Mukhanov et al.(1992) yields the Poisson equation of the Newtonian Theory of Gravity only if the Hubble function, i.e., the expansion scalar, vanishes. This, however, violates the background Friedmann equation, as will be shown in Section7. In fact, the background Friedmann equation is needed to derive the Poisson equation from the linearized Friedmann equation in the non- relativistic limit. Consequently, the gauge-invariant quantities defined by Mukhanov et al.(1992) are not equal to the true physical perturbations. In the approach of Ellis and Bruni(1989); Ellis et al.(1989); Ellis and van Elst(1998) density pertur- bations are defined using gradients. They define a gauge-invariant and covariant function which ‘closely 3 corresponds to the intention of the usual gauge-dependent density contrast function’. However, a pertur- bation theory based on this definition does not yield the Poisson equation in the non-relativistic limit. The above mentioned pioneering treatments caused an abundance of other important works, e.g., Malik and Wands(2009); Knobel(2012); Peter(2013); Malik and Matravers(2013); Kodama and Sasaki (1984); Ma and Bertschinger(1995). The review article of Ellis(2017) discusses the work of Lifshitz and Khalatnikov, and also provides an overview of other research on the subject. From the vast literature on cosmological density perturbations, it must be concluded that there is still no agreement on which gauge-invariant quantities are the true energy density perturbation and particle number density perturbation. None of the cosmological perturbation theories in the literature has a correct non-relativistic limit as explained in Section7, so that there is still no solution to the gauge problem of cosmology. 1.3 Solution of the Gauge Problem of Cosmology phys phys As said before, the true, physical, density perturbations "(1) and n(1) are hidden in the general solution of the linearized Einstein equations and conservation laws and it has proved difficult to determine in advance which linear combination of gauge-dependent quantities are equal to these true density perturbations. To solve this problem, the decomposition theorems of York, jr.(1974), Stewart and Walker(1974) and Stewart(1990) for symmetric three-tensors are used. In the literature, these decomposition theorems are only applied to the perturbed metric three-tensor. This, however, is not sufficient. The decomposition of the perturbed Ricci three-tensor must also be taken into account. Only then the linearized momentum constraint equations are consistent with the decomposition of the fluid three-velocity into a rotational part and a divergence part. This makes it possible to decompose the linearized equations into three independent systems which govern the evolution of scalar perturbations, vector perturbations and tensor perturbations. Only the part of the perturbed Ricci three-scalar which is associated with scalar perturbations is non-zero, so that only scalar perturbations are coupled to "(1) and n(1).
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